A sex-determination system is a biological system that determines the development of sexual characteristics in an organism. Most common sex-determination systems in animals involve a genetic mechanism based on the chromosomes of that organism. However, other systems can involve other variables such as temperature. The details of some sex-determination systems are not yet fully understood.

The most familiar sex-determination system is the XY sex-determination system found in human beings and other mammals. In the XY sex-determination system, females have two of the same kind of sex chromosome (XX), while males have two distinct sex chromosomes (XY). Some species (including humans) have a gene SRY on the Y chromosome that determines maleness; others (such as the fruit fly) use the presence of two X chromosomes to determine femaleness.
There can also be other, less common, cases: see the "chromosome" article for information on abnormalities of the XY sex-determination system, and the "intersexual" article for information on variations in human sexual forms.

SRY is not the only male-determining gene in mammals, or even the most common: most non-primate mammals use a different Y-chromosome gene, UBE1, for this purpose. Also, two species of "mole voles", Ellobius[?] tancrei and E. lutescens, have lost the Y chromosome entirely. In one species, both sexes have unpaired X chromosomes; in the other, both females and males have XX.

The WZ sex-determination system is found in birds and some insects and other organisms. In the WZ sex-determination system, the situation is reversed: females have two different kinds of chromosomes (WZ), and males have two of the same kind of chromosomes (ZZ).

The haploid-diploid system is found in Hymenoptera. Males are haploid; females are diploid. Thus, if a queen bee mates with one drone, her daughters share 3/4 of their genes with each other, not 1/2 as in the XY and WZ systems. This is believed to be significant for the development of eusociality, as it increases the significance of kin selection.

Many other exotic sex-determination systems exist. In some species of reptiles, including alligators, sex is determined by the temperature at which the egg is incubated. Other species, such as some snails, practice sex change: adults start out male, then become female.

Some species have no sex-determination system. Earthworms and some snails are hermaphrodites; a few species of lizard, fish, and insect are all female and reproduce by parthenogenesis.

In some arthropods, sex is determined by infection. Bacteria of the genus Wolbachia[?] alter their sexuality; some species consist entirely of ZZ individuals, with sex determined by the presence of Wolbachia.